Quantification of XPA gene expression levels in human and mouse cell lines by competitive RT-PCR.

Expression of the mammalian photoproduct-specific DNA-binding protein XPA has been quantified by competitive reverse transcription-polymerase chain reaction, a method that provides relative numbers of mRNA molecules per cell. Human primary and simian virus (SV)40-transformed fibroblasts had 4.8 and 8.4 transcripts per cell, respectively; mouse embryonic and SV40-transformed fibroblasts had 6.7 and 5.5 transcripts per cell, respectively. None of these differences are significant, and the mean value of 5 to 8 transcripts per cell indicates that XPA is expressed as a low-abundance mRNA. Two cell lines transfected with XPA on a conditional promoter showed different numbers of XPA mRNA molecules, consistent with their respective responses to an inducer and their sensitivity to ultraviolet light. The similarity of results in human and mouse cells shows that a difference in XPA expression cannot account for the greater repair of nontranscribed DNA in human cells.